Section IV. Stormwater Management

A. General

The City of Aurora Stormwater Ordinance is a modification of the Kane County Stormwater Ordinance (hereto referred as "Stormwater Ordinance"). The City Engineer is the Administrator of the Stormwater Ordinance. In addition to the Stormwater Ordinance, the City has developed additional ordinances and standards, including these Standard Specifications, in order to provide additional criteria for minimum design standards. In the event that any conflict should arise between the Stormwater Ordinance and any other City Ordinances or standards, the Stormwater Ordinance shall govern, unless otherwise determined by the Administrator.

Properties within a 5 mile radius of the Aurora Airport are subject to the planning recommendations of the Federal Aviation Administration's Advisory Circular Number 150/5200-33A regarding "Hazardous Wildlife Attractants on or Near Airports." In an effort to accomplish the FAA planning recommendations, all stormwater management facilities shall have a minimum bottom slope of 1.5% established with deep-rooted native plantings with no permanent open water feature, to deter waterfowl from utilizing the facility and to promote stormwater infiltration. In addition, the retention volume component of the Stormwater Ordinance will not be required, by utilizing the suggested design requirements of Section 203g of the Kane County Technical Manual. It is understood that project design constraints are unique and therefore each project must be reviewed based upon site-specific conditions.

B. Release Rate

Release Rates shall be in accordance with the Stormwater Ordinance. The drainage system for a property shall be designed to control the peak rate of discharge from the property for the 100-year, 24-hour event to levels which will not cause an increase in flooding or channel instability downstream when considered in aggregate with other developed properties and downstream drainage capacities.

Tail water conditions from the downstream drainage system shall be considered for the design of the outlet structure.

C. Detention Requirements

Volume Requirements

The detention storage shall be computed by using the Event Hydrograph Routing Method and the City of Aurora's Modified Rational Method as shown in Exhibit IV-1 (PDF). Whichever method requires the greatest detention volume, shall govern. The Event Hydrograph Routing Method shall be in accordance with the parameters set forth within the Kane County Ordinance. The existing and proposed conditions models shall both be a critical duration analysis (each duration starting at 30 minutes to 240 hours utilizing the rainfall table below).

Within the Indian Creek and Blackberry Creek watersheds, the required stormwater storage volume shall be 110% of the volume required by the foregoing provisions of this section.

Design Rainfall

All design analysis for minor and major stormwater systems and detention shall incorporate point frequency rainfall intensities as listed by the "Illinois State Water Survey (ISWS) Bulletin 70 Appendix A Point Frequency Distributions for the Aurora Station." The table below represents the rainfall depth (in) from Appendix A for the 100-year frequency and the rainfall intensities for each time duration (see Section III-A for a table indicating the 10 year frequency).

Duration

Rainfall Depth

Intensity (Inches per Hour)

5 minutes

1.01 inches

12.12

10 minutes

1.85 inches

11.10

15 minutes

2.27 inches

9.08

30 minutes

3.11 inches

6.22

1 hour

3.95 inches

3.95

2 hours

4.96 inches

2.48

3 hours

5.38 inches

1.79

6 hours

6.30 inches

1.05

12 hours

7.31 inches

0.61

18 hours

7.73 inches

0.43

24 hours

8.40 inches

0.35

48 hours

9.07 inches

0.19

72 hours

9.80 inches

0.14

120 hours

11.05 inches

0.09

240 hours

12.6 inches

0.05

Positive Drainage

An overland flood route shall be labeled on the plans and calculated to convey all the stormwater runoff for the major 100-year rain event to the proposed detention basin assuming the minor drainage system (storm sewer, inlets and catch basins) are not functioning. Roadways may be used to convey the overland flow route and shall have a maximum flow depth at the crown of 6 inches and in no case shall the depth of flow be greater than 12 inches.

All areas of the property must provide an overland flow path that will pass the 100-year flow. If the tributary area is less than 20 acres, the vertical distance between the 100 year high water level within the overland flow path and the lowest building opening shall be a minimum of 1 foot. If the tributary area is greater than 20 acres, the 100-year high water level shall be at a stage at least 2 feet below the lowest opening of any building in the vicinity of the flow path in accordance with the Kane County Stormwater Technical Manual section T201 (d).

Restrictor Catch Basin

Restrictor Catch Basin shall conform to Exhibit IV-C-4A (PDF) or Exhibit IV-C4B (PDF) and shall be located to be accessible by heavy vehicles. Preferably, the access shall be from a paved surface. However, in the event that the restrictor is off pavement, a 10 feet wide paved path must be installed from the closest hard surface to the restrictor catch basin to allow heavy vehicular access and shall be subject to the approval of the City Engineer and the Director of Water and Sewer Maintenance Division.

Underground Detention

Underground Detention is not permitted.

Basement Floors

Basement Floors shall be a minimum of 2 feet above the Normal Water Level of any nearby stormwater detention facility.

Minimum Detention Outlet Size

The minimum size for a single outlet storm detention release pipe shall be 12 inches RCP with a minimum restrictor size of 2 inches SDR 26. If restrictor size is greater than 12 inches then the outlet pipe must be at least one size larger than restrictor.

Restrictor Type

All restrictors shall be a pipe restrictor that is grouted-in-place. The restrictor cannot be the outlet storm sewer. Weirs within storm structures are not acceptable.

Parking Lots

The maximum stormwater ponding depth in any parking area shall not exceed one foot for more than 4 hours.

Safety Considerations

The drainage system components, especially all detention basins, shall be designed to protect the safety of any person coming in contact with the system, at any time.

Restrictor Sizing

Restrictors shall be sized for the required release rate of 0.10 cfs per tributary acre or the pre-developed release rate (which ever is the lesser rate) at the high water elevation of the required storage. For areas within the Blackberry Creek and Indian Creek watersheds, the restrictor shall be sized as stated above, however, the overflow weir shall be set to provide an additional 10% of stormwater storage volume.

Accommodating Flows From Upstream Tributary Areas

Stormwater runoff from areas tributary to the property shall be considered in the design of the property's drainage system. Whenever practical, flows from upstream areas that are not to be detained should be routed around the proposed basin. When stormwater runoff from an upstream area cannot be routed around the proposed basin, the proposed restrictor must be sized to pass the developed release rate (0.10 cfs per acre) from the on-site and off-site tributary areas.

Overflow Structures

All stormwater detention basins shall be provided with an overflow structure capable of safely passing excess flows and shall be in compliance with Exhibit IV-C-2 (PDF) and Exhibit IV-C-3 (PDF). The overflow weir shall be designed to convey the proposed critical duration 100-year peak flow entering the basin, shall not exceed 1 foot of depth and shall have positive downstream conveyance. Once the width and elevation of the weir is established, the basin shall have a minimum grade surrounding the basin 1 foot above the weir elevation (1 foot of freeboard above the weir elevation). The overflow structure shall have minimum width of 5 feet.

Setback Requirements

Distance From the Right-of-Way
The minimum setback from the Right of Way shall be as shown in Exhibit IV-C-5 (PDF).

Setback Between the High Water Level of the Detention Basin & the Building/Lot
All residential development shall comply with the more restrictive of the following requirements:

The High Water Level plus 1 foot of freeboard shall not encroach onto a residential lot, or

The minimum distance between the residential building envelope and the High Water Level shall be 15 feet.

Detention in Floodplains

See Section 203 (i) of the Kane County Stormwater Ordinance.

Early Completion of Detention Facilities

Where detention, retention, or depressional storage areas are to be used as part of the drainage system for a property, they shall be completed as the first element of the initial earthwork program. This is to ensure that if soil erosion and sediment control practices are not adequately implemented, any eroded sediment will be captured in these areas and will have to be removed by the developer before project completion.

D. Wet Detention Basin Design

Wet detention basins shall be designed to remove stormwater pollutants, so as to be safe, aesthetically pleasing, and to provide recreational use whenever feasible.

Wet Basin Depths

Wet basins shall be at least 5 feet deep, excluding near shore banks and safety ledges and shall be at least 10 feet deep over 25% of the normal water level surface area.

Wet Basin Shoreline Slopes

The side slopes of wet basins from the safety ledge to the high water elevation shall not be steeper than 4 to 1 (horizontal to vertical).

Safety Ledges

All wet detention basins shall have a level safety ledge at least 10 feet in width and 3 feet below the Normal Water Level.

Desiltation Basins

E. Dry Detention Basin Design

Velocity Dissipation

Velocity dissipation measures shall be incorporated into dry basin designs to minimize erosion at inlets and outlets and to minimize the re-suspension of pollutants.

Inlet and Outlet Orientation for Both Wet & Dry Detention Basins

To the extent feasible, the distance between detention inlets and outlets shall be maximized. If possible, they should be at opposite ends of the basin.

Retention

As per the Kane County Stormwater Ordinance, retention volume should be provided below the outlet elevation for all dry detention basins. In cases where the retention component of the facility is not occupying the entire bottom of the basin, the minimum ground slope shall be 2%. The retention area must be planted with species that are tolerant to inundation.

F. Drainage and Stormwater Management Report Submittal

The drainage and stormwater report shall include engineering drawings and supporting calculations describing the existing and proposed drainage system as required by the Stormwater Ordinance.

The City of Aurora's Rational Method for sizing the detention facility shall be submitted with supporting calculations for the runoff coefficient.

The following shall be submitted in addition to the information required under the Kane County Stormwater Ordinance:

A drainage area map indicating the tributary areas and sub-drainage basins within the development.

A schematic showing the locations of the Structures and Reaches numbered according to the input data.

A list of input data, runoff volume, and summary table.

A disk shall be supplied containing all input and output files.

Time of concentration calculation in accordance with and formatted to TR-55.

Supporting documentation for the "CN" value, including a copy of the Soils Survey map classifying the soil type.

Input and output data proving that the proposed detention facility will not release more than the existing condition release rate when the restrictor is clogged.